Solar cycle 20

Solar cycle 20 was the twentieth solar cycle since 1755, when extensive recording of solar sunspot activity began.[1][2] The solar cycle lasted 11.4 years, beginning in October 1964 and ending in March 1976. The maximum smoothed sunspot number (SIDC formula) observed during the solar cycle was 156.6 (November 1968), and the starting minimum was 14.3.[3] During the minimum transit from solar cycle 20 to 21, there were a total of 272 days with no sunspots.[4][5][6]

Solar cycle 20
Solar magnetogram from solar cycle 20 (1974).
Sunspot data
Start dateOctober 1964
End dateMarch 1976
Duration (years)11.4
Max count156.6
Max count monthNovember 1968
Min count14.3
Spotless days272
Cycle chronology
Previous cycleSolar cycle 19 (1954–1964)
Next cycleSolar cycle 21 (1976–1986)
One of the largest solar flares ever recorded, from solar cycle 20 (19 December 1973).

Comparison with other cycles shows that geomagnetic activity during the declining phase of cycle 20 (1973–1975) was unusually high.[7]

Data from solar cycle 20 was used to build the K-1974 solar proton fluence model, used for planning space missions during solar cycle 21.[8]

1972
Main article: Solar storm of August 1972

An extremely active sunspot region, McMath 11976, produced a historic series of flares and CMEs in July, one CME transited to Earth in a record low 14.6 hours and produced a strong geomagnetic storm that caused widespread electrical and communications grid disturbances, and notably, the accidental detonation of numerous U.S. Navy magnetic sea mines in North Vietnam.[9]

See also

References
  1. Kane, R.P. (2002), "Some Implications Using the Group Sunspot Number Reconstruction", Solar Physics, 205 (2): 383–401, Bibcode:2002SoPh..205..383K, doi:10.1023/A:1014296529097
  2. "The Sun: Did You Say the Sun Has Spots?". Space Today Online. Retrieved 12 August 2010.
  3. SIDC Monthly Smoothed Sunspot Number. ""
  4. Spotless Days. ""
  5. Dr. Tony Phillips (11 July 2008). "What's Wrong with the Sun? (Nothing)". NASA. Archived from the original on 14 July 2008.
  6. Solaemon's Spotless Days Page. ""
  7. Gosling, J. T.; Asbridge, J. R.; Bame, S. J. (1 August 1977). "An unusual aspect of solar wind speed variations during solar cycle 20". Journal of Geophysical Research. 82 (22): 3311–3314. Bibcode:1977JGR....82.3311G. doi:10.1029/JA082i022p03311.
  8. Miroshnichenko, Leonty (2001). Solar Cosmic Rays. Springer. p. 395. ISBN 0792369289.
  9. Knipp, Delores J.; B. J. Fraser; M. A. Shea; D. F. Smart (2018). "On the Little‐Known Consequences of the 4 August 1972 Ultra‐Fast Coronal Mass Ejecta: Facts, Commentary and Call to Action". Space Weather. 16: 1635–1643. Bibcode:2018SpWea..16.1635K. doi:10.1029/2018SW002024.


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